In wireless communication systems, the use of antenna arrays at the base station has been shown to increase both range, through increased gain, and capacity, through interference suppression. With adaptive antenna arrays, the signals received by multiple antenna elements are weighted and combined to improve system performance, e.g., by maximizing the desired receive signal power and/or suppressing interference. The performance of an adaptive antenna array increases dramatically with the number of antennas. Referring to an article entitled, "The Impact of Antenna Diversity on the Capacity of Wireless Communication Systems," by J. H. Winters, R. D. Gitlin and J. Salz, in IEEE Trans. on Communications, April 1994, it is shown that using an M element antenna array with optimum combining of the received signals can eliminate N.ltoreq.M-1 interferers and achieve an M-N fold diversity gain against multipath fading, resulting in increased range.
Most base stations today, however, utilize only two receive antennas with suboptimum processing, e.g., selection diversity where the antenna having the larger signal power is selected for reception and processing. It is desirable to be able to modify existing base stations to accommodate larger arrays of antennas and/or improved received signal combining techniques. However, modifying existing equipment is difficult, time consuming, and costly, in particular since equipment currently in the field is from a variety of vendors.
One alternative is to utilize an applique, which is an outboard signal processing box, interposed between the current base antennas and the input to the base station, which adaptively weights and combines the received signals fed to the base station, optionally utilizing additional antennas. FIG. 1 shows a base station utilizing an applique. A key to the viability of utilizing the applique approach is that it should require little, if any, modification of the base station equipment. This constraint implies that the processing performed by the applique must be transparent to the existing equipment. Ideally, the signal emerging from the applique should appear to the existing base station as a high-quality received signal from a single antenna.
A difficulty in achieving this is caused by the soft decision decoding which occurs in many base stations. With soft decision decoding, the received symbols are estimated using convolutional decoding based on path metrics dependent on both the phase error per symbol and the amplitude. The amplitude depends on the fading envelope of the received signal, which varies at a rate depending on the carrier frequency and the speed of a mobile unit, e.g., up to 184 Hz for a Personal Communication Service (PCS) system at 2 GHz. It is not desirable to perform the decoding in the applique, as the decoding and re-encoding of the output signal requires significant complexity and introduces additional delay. For example, in the North American digital mobile radio standard IS-136, interleaving is present on the transmitted signal, both within a time slot and between time slots, and thus decoding introduces at least an additional frame delay. However, if the applique does not perform decoding, but detects the received symbols, uses these detected symbols to remodulate the carrier, and sends the remodulated carrier signal to the base station for decoding, then the soft decision information is lost, resulting in degraded performance, e.g., a loss of the 2 dB gain due to soft decision decoding at a 10.sup.-2 bit error rate.
In order to avoid the loss of coding gain, it is insufficient to simply send the remodulated array output signal, i.e., the weighted and combined array output signal without data detection, to the base station, because standard adaptive array algorithms determine the weights that adjust the array output signal to closely match a reference signal, whose amplitude is typically constant and does not vary with the received signal level. In this case, the array output signal will have the phase error per symbol needed for soft decision decoding, but the weighted output signal will have nearly constant amplitude, at the sampling instant. Thus, the output signal has a noise level which varies with the fading (and a constant desired signal level) rather than a signal level that varies with the fading (and a constant noise level) which is needed for soft decision decoding.
A method in accordance with the present invention for performance improvement of a digital wireless receiver is described.